Rhododendron Blossom Diseases

Rhododendron Diseases in General

• Root rot and stem dieback are fungal diseases that attack the roots or stems of this plant. The organisms that cause these diseases often occur in overly wet soils, which points up the importance of growing your rhododendrons in well-drained areas. Powdery mildew is a common fungal disease of many types of plants. Providing your rhododendron with adequate air circulation is helpful in preventing mildew, but plants in coastal areas tend to become infected more frequently than plants grown inland. Rust is a disease that causes reddish-orange pustules on lower leaf surfaces. Some species are resistant to rust.

Bud Blight

• When bud blight strikes a rhododendron, young flower buds become brown and grow black bristles called coremia, which are fungus spores of the pathogen Briosia azaleae. Buds that contract this disease never open. Sometimes frost can damage flower buds and cause them to turn brown, so be careful to examine the buds on your plant for the black bristles if you suspect that this disease is present.

Flower Blight

• When the disease called flower blight or ovulinia petal blight strikes a rhododendron, the flowers develop watery looking spots that are small and pale or reddish, and then they turn soft and brown. Infected areas grow rapidly, causing rotting of entire flower heads in short order, such as two or three days. This disease happens most often in warm, rainy weather, and is more common in the Southeastern region of the United States.

Controlling Rhododendron Flower Diseases

• If your rhododendron is suffering from bud blight, snip off the buds that appear to be affected. Destroy them by placing them in a black plastic garbage bag instead of adding them to your compost pile or using them as mulch around your plant. The Fraser South Rhododendron Society’s website states that using a fungicide spray is not always necessary because this disease rarely becomes widespread or serious enough to warrant such control methods. Some researchers have implicated the rhododendron leafhopper in spreading this disease, so if this insect is present in your yard, control can help to prevent infection.

  If flower blight attacks your rhododendron, keep the area clean by removing all dead flowers and leaf litter from the ground around your plant. The chemicals benomyl or triadimefon are effective in controlling flower blight, according to the Fraser South Rhododendron Society’s website. The University of North Carolina’s website recommends spraying the ground under your rhododendron with 20 percent Terraclor. Spray plants with triadimefon when flower buds start to show color. Always pick and destroy infected flower heads.